1. Field of the Invention
The present invention relates to a fuel cell, and more particularly to a fuel cell which is capable of efficiently controlling the temperature of a fuel cell stack.
2. Description of the Related Art
Phosphoric acid fuel cells have a plurality of generator cells each comprising an electrolyte electrode assembly having an electrolyte of a matrix made of a basic polymer such as polybenzimidazole or a porous silicon carbide and impregnated with a concentrated phosphoric acid, the electrolyte being interposed between an anode electrode and a cathode electrode, and a pair of separators sandwiching the electrolyte electrode assembly therebetween. A certain number of such generator cells are stacked and electrically connected in series, making up a fuel cell stack which is stored in a container. The fuel cell stack stored in the container serves as a phosphoric acid fuel cell.
A generator cell for solid polymer fuel cells employs an electrolyte comprising a polymer ion exchange film (positive ion exchange film) made of polytetrafluoroethylene sulfonic acid or the like and impregnated with water. A certain number of such generator cells are also stacked and electrically connected in series, making up a fuel cell stack which is stored in a container. The fuel cell stack stored in the container serves as a solid polymer fuel cell.
Each of the anode and cathode electrodes has a gas diffusion layer made of a carbon cloth or the like and an electrode catalyst layer made of porous carbon particles carrying platinum on their surfaces and uniformly disposed on the surface of the gas diffusion layer. The anode and cathode electrodes are joined to the electrolyte such that their electrode catalyst layers confront each other through the electrolyte. The anode and cathode electrodes of the type described above can be used in various kinds of fuel cells in addition to the phosphoric acid fuel cells and the solid polymer fuel cells.
For operating the fuel cells thus constructed, a fuel gas such as a gas chiefly containing hydrogen (hereinafter referred to as xe2x80x9chydrogen-containing gasxe2x80x9d) is supplied to the anode electrode, whereas an oxidizing gas (hereinafter referred to as xe2x80x9coxygen-containing gasxe2x80x9d) is supplied to the cathode electrode. The hydrogen in the hydrogen-containing gas is ionized as represented by the following formula (A), generating hydrogen ions and electrons:
2H2xe2x86x924H++4exe2x80x83xe2x80x83(A)
The generated hydrogen ions move through the electrolyte to the cathode electrode. During this time, the electrons flow to an external circuit that is electrically connected to the anode electrode and the cathode electrode, are used as an electric energy in the form of a direct current to energize the external circuit, and then flow to the cathode electrode.
The hydrogen ions that have moved to the cathode electrode and the electrons that have moved to the cathode electrode via the external circuit react with the oxygen contained in the oxygen-containing gas supplied to the cathode electrode, as indicated by the following formula (B):
O2+4H++4exe2x86x922H2Oxe2x80x83xe2x80x83(B)
In order to enable the fuel cell to exhibit an excellent cell performance, the efficiency of the reactions represented by the above formulas (A), (B) needs to be high during the operation of the fuel cell. The fuel cell incorporates a cooling mechanism for setting the temperature of the fuel cell stack to make the above reactions most efficient. For example, such a cooling mechanism comprises coolant passages defined in the separators for cooling the generator cells with a coolant such as water or the like which is supplied to the coolant passages.
However, the above cooling mechanism is disadvantageous in that a temperature distribution tends to occur within the end faces of the generator cells. To avoid this drawback, there has been proposed a through cooling arrangement for cooling the generator cells with a coolant that flows through coolant passages which are defined through the generator cells in the direction of the stack of the generator cells, as disclosed in Japanese laid-open patent publications Nos. 61-131370 and 2000-90943.
The coolant passages are usually made up of an electric insulator, as disclosed in Japanese laid-open patent publication No. 2000-90943, because if the coolant passages were made up of an electric conductor, the anode and cathode electrodes would be short-circuited to each other.
The material of the coolant passages is also required to be corrosion-resistant and heat-resistant and have a gas sealing capability. One typical material which exhibits such properties is ceramics.
However, the ceramics used as the material of the coolant passages and the porous carbon or metal used as the material as the separators have different coefficients of thermal expansion. Consequently, the flow passages and the generator cells that are held in intimate contact with each other at the room temperature become spaced from each other when their temperature is increased to operate the fuel cell. When the flow passages and the generator cells are spaced from each other, since the heat from the generator cells is not transferred to the coolant, the cooling efficiency of the fuel cell stack is greatly reduced. With the through cooling arrangement, therefore, the temperature of the fuel cell stack cannot efficiently be controlled.
It is therefore an object of the present invention to provide a fuel cell which is capable of efficiently controlling the temperature of a fuel cell stack even though a temperature regulating medium is passed in the direction in which generator cells are stacked.
To achieve the above object, there is provided in accordance with the present invention a fuel cell comprising a fuel cell stack including a plurality of generator cells stacked and electrically connected in series, each of the generator cells comprising an electrolyte electrode assembly having an anode electrode for being supplied with a fuel gas, a cathode electrode for being supplied with an oxidizing agent gas, and an electrolyte interposed between the anode electrode and the cathode electrode, and a pair of separators having a fuel gas supply path for supplying the fuel gas and an oxidizing agent gas supply path for supplying the oxidizing agent gas, and sandwiching the electrolyte electrode assembly, the fuel cell stack having a through hole defined therein in a direction in which the generator cells are stacked, and a tube inserted through the through hole, the tube having a passage for passage of a temperature regulating medium therethrough, the fuel cell stack including at least one of an insulating elastic body and an insulating lubricant interposed between the tube and an inner wall of the through hole.
If the temperature of the fuel cell exceeds the operating temperature thereof while the fuel cell is in operation, then a cooling medium is introduced into the passage in the tube to cool the fuel cell. At this time, the gap between the generator cells and the tube is increased. However, the insulating elastic body is expanded by way of elastic deformation, or the insulating lubricant is extended, closing the increased gap. Thus, any spaces are prevented from being created between the generator cells and the tube. The heat of the fuel cell stack is quickly transferred to the tube through the insulating elastic body or the insulating lubricant for thereby efficiently cooling the fuel cell stack. For increasing the temperature of the fuel cell stack, a heating medium may be passed through the passage in the tube.
Since any spaces are prevented from being created between the generator cells and the tube, the temperature of the fuel cell stack and hence the fuel cell can be controlled efficiently.
The insulating elastic body or the insulating lubricant should preferably have a thermal conductivity of at least 0.5 W/mxc2x7K for efficiently transferring the heat.
The tube should preferably be made of metal. Since metal generally has a high thermal conductivity, the heat is transferred highly efficiently through the tube. Because at least one of the insulating elastic body and the insulating lubricant is present on the surface of the tube, the anode and cathode electrodes of each of the generator cells are prevented from being short-circuited to each other even though the tube is made of metal.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.